How CPI, NFP and the Fed Move Markets: the Reaction Function

Markets don't move on whether economic data is "good" or "bad" — they move on the gap between the data and what was already expected, filtered through how the central bank is likely to respond. In short, the surprise is the fuel and the Fed's reaction function is the engine. This guide explains the mechanism in plain English, so a strong jobs report sending stocks down stops being a mystery.

The counterintuitive first rule: markets trade the surprise

The single most useful idea here is that expectations are already in the price. Before any major release, analysts publish forecasts, and traders position for them. If the number lands exactly where expected, the news is already "priced in" and the reaction can be muted — even if the number itself looks dramatic.

What moves markets is the surprise: the distance between the actual figure and the consensus expectation.

• Data that comes in hotter than expected is a surprise in one direction.
• Data that comes in cooler than expected is a surprise in the other.
• A number that matches the forecast, however big, is often a non-event.

This is why "good news" and "bad news" are the wrong lens. The right question is always: good or bad relative to what the market already assumed?

The three releases that dominate the calendar

A handful of data points carry outsized weight because they speak directly to the central bank's job:

• CPI (inflation). The Consumer Price Index measures how fast prices are rising. It matters because taming inflation is the central bank's primary mandate. A hotter-than-expected CPI raises the odds of tighter policy; a cooler one lowers them.
• NFP (jobs). Non-Farm Payrolls measures how many jobs the US economy added. A strong labour market can mean more spending and more inflationary pressure; a weak one points the other way.
• The Fed itself. Rate decisions, the accompanying statement, the press conference and officials' speeches don't just report on the economy — they reveal how the central bank is thinking, which is what markets most want to know.

These releases are watched so closely because they feed directly into the one thing that sets the price of money: the policy rate.

The reaction function: the engine that translates data into moves

Here's the piece that ties it together. The reaction function is the market's model of how the central bank will respond to incoming data. Every surprise gets run through that model:

Hotter inflation or a stronger labour market → higher odds the central bank keeps policy tight or tightens further → higher expected interest rates.

Cooler inflation or a weaker labour market → higher odds the central bank eases → lower expected interest rates.

Because interest rates are the price of money — the discount rate underneath every asset — a shift in expected rates ripples outward: into bond yields first, then the dollar, then equities, commodities and everything else. The data is the input; the reaction function is what converts it into a market move.

Why "good news is bad news" happens

Now the puzzle resolves. Picture a very strong jobs report arriving during a period when the central bank is fighting inflation:

• The economy looks healthy — ordinarily "good news."
• But a hot labour market raises the risk of persistent inflation.
• So the market lifts its expectation of how high rates will go, or how long they'll stay high.
• Higher expected rates pressure bond prices and can weigh on stocks.

The result: strong data, falling markets. It isn't perverse — it's the reaction function at work. Whether "good news" is good for markets depends entirely on what the central bank is worried about at the time. In an inflation fight, strength can be unwelcome; in a growth scare, strength can be a relief.

How to actually use this

You don't need to forecast the data. You need a simple three-step read around each release:

1. Know the expectation, not just the number. The move lives in the gap between actual and consensus. Check both.
2. Know the regime. Is the central bank most worried about inflation or about growth? That determines how a given surprise will be interpreted.
3. Watch rates first. Bond yields and rate expectations usually react most directly. From there the impulse spreads to the dollar, equities and commodities.

When someone asks "why did the market jump on that number?", the useful answer is almost always: it surprised versus expectations, and here's how that changes the rate path. A reaction with a reason beats a headline alone.

What breaks this framework

No market relationship is a law, and honesty about the exceptions is part of using it well:

• The regime shifts. The same CPI surprise can be read very differently depending on whether the central bank's focus is inflation or recession risk. The reaction function isn't fixed.
• Details beat headlines. Markets sometimes look past the top-line number to the composition — which parts of inflation, which parts of the jobs report — and react to those instead.
• Revisions muddy the water. Big data are revised later. A number that moved the market can be quietly changed, complicating the original reaction.
• Competing events. A bigger story — a geopolitical shock, a liquidity event — can swamp a data release entirely, so the "textbook" reaction never appears.

When a release lands and markets don't react the way the surprise suggests they should, that's not noise — it usually means the regime, the details, or a competing event has changed the interpretation. Noticing the non-reaction is as valuable as knowing the rule.

The one-line takeaway

Don't ask whether the data was good or bad. Ask how far it surprised, and how the central bank is likely to respond — that's the mechanism behind almost every data-driven move.